GPS accuracy

The

The latest marathon, 44.21 km.

I am so frustrated with this watch! Did anyone ever find satisfactory solution?

It was over 2 miles over for my marathon (in the middle of nowhere, not a big city). Since then I've been paying closer attention to GPS accuracy and usually pretty off and it looks like my 3 year old drew the map. The last several months I've wearing my old Forerunner 220 as well to compare results and the 220 is way more accurate. Garmin sent me a new 645M last week which I really appreciated. I changed all the settings again that they recommended and it's still so off. After 10 years of loving Garmin I suddenly have major trust issues and not sure what to do next.. I'm tired of wearing 2 watches and it's just a matter of time before my 220 dies. First world problems, I know. 

so far, I use GPS + GLONASS and my 645M is spot on, boy I hope it stays that way!

I posted a somewhat in depth comment on the forum a while ago where I compared the result of different GPS-settings on the Forerunner 645.
I also compared my new-ish Garmin to my older watches (including other brands).
However, the comment was flagged as “abuse” and removed by the admins. So I better not share my findings again. ;)

My own homebrewed theory is that some older watches only sampled the location every 5 seconds, while newer watches sample the location at least every second, this results in a more jagged track that over time measures distance less accurate. However, some of the most crazy inaccuracies on my new watch must be down to something else, perhaps the new GPS chip from Sony that everyone (including Garmin) has switched to in recent years. …this, however, does not explain why the altimeter is even more off than the GPS. ;)

With all of that said; I still get the feeling that the GPS accuracy is a lot better now than a year ago, so they must have done something in the software.

I totally could've written the same post that RunStephRun9 wrote, except I haven't run a marathon lately.   But yep, 645 is off, 220 works great but is going to die one of these days, and I'm not trusting the 645 at all.  And yes, I have definitely thought "first world problems" as well.  But dang it, I bought a new Garmin and I want it to work properly.  I don't want it always telling me that I'm running anywhere from 5 - 15 seconds per mile faster than I am so that when I come to race day I fail miserably because I've actually been training at a slower pace than I thought.  I've had 2 645's so far.  I'm going to see what I can do to exchange for a 245.  I'd rather have accuracy than other random functions.

The altimeter is not more off than the GPS. The altimeter works perfectly. The problem is still the GPS. When the watch lost the GPS fix during activity, it stops refreshing data from altimeter until the GPS is back. Therefore, elevation data is incorrect at the end of activity.

David said:

The altimeter is not more off than the GPS. The altimeter works perfectly. The problem is still the GPS. When the watch lost the GPS fix during activity, it stops refreshing data from altimeter until the GPS is back. Therefore, elevation data is incorrect at the end of activity.

For me altimeter doesn't work perfectly, but haven't noticed at GPS is lost then.

David said:

When the watch lost the GPS fix during activity, it stops refreshing data from altimeter until the GPS is back. Therefore, elevation data is incorrect at the end of activity.

Will you explain what you're experiencing in more detail?  The barometric altimeter works independently of GPS.  It should be working 24/7 and not just when you have a GPS fix.